Organometallic halide perovskites have rapidly attracted increasing interest for use as photoactive materials in photovoltaic applications. Within only a few years, the power conversion efficiency (PCE) of perovskite solar cells has undergone an unprecedented rapid rise, from the first reported PCE value of 3.8% in 2009 to the certified highest value of 20.1% in 2014 (M. A. Green et al, Prog. Photovoltaics Res. Appl. 2015, 23, 1). Depending on the location of the electron transport layer (ETL) and the hole transport layer (HTL), one of two device architectures is generally employed: either the ‘normal’ substrate/electrode/HTL/perovskite film/ETL/electrode or the ‘inverted’ substrate/electrode/ETL/perovskite film/HTL/electrode. Of these the inverted architecture has drawn the most interest in recent years. This can be explained by a higher quality of perovskite film formation on the traditional TiO2 ETL layer in inverted devices, compared to the traditional PEDOT:PSS HTL layer in normal devices; leading to improved device characteristics.
Currently, the TiO2 ETL layer in these devices is generally created by means of electron beam deposition, but this is a slow and costly process to use on an industrial scale. Methods for solution processing of the TiO2 layer exist, but the obtained layers cannot resist the subsequent processing of the perovskite material on top of them and/or require very high temperature thermal annealing (typically 500-600° C.). Such annealing temperatures are too high for some electrodes such as indium tin oxide. This lack of resistance to subsequent processing of the perovskite material is at least partly due to the fact that both the solution processing of TiO2 and of the perovskite make use of polar solvents. As a consequence, in order to prevent damages to the TiO2 layer in presence of the perovskite-forming solution, the TiO2 layer needs to be sufficiently thick, hard and dense; this is typically not the case for solution processed TiO2 layers. The use of very high annealing temperatures for the TiO2 layer is also undesired, as it limits the range of electrodes that can be used and it is incompatible with flexible substrates or Si-multi-junction devices.
As the rest of the perovskite device can be made through simple solution processing or thermal evaporation steps, the creation of the TiO2 layer thus becomes a limiting step. Hence, a need exists for a solution processed ETL with high electrical conductivity, which can resist the processing of the perovskite and does not require high temperatures for its production.